1. Field of the Invention
This invention relates to cellular telephone systems and, more particularly, to processes for quantifying the performance of CDMA cellular telephone systems.
2. History of the Prior Art
Presently available commercial mobile communication systems typically include a plurality of fixed base stations (cells) each of which transmits signals to and receives signals from mobile units within its communication area. Each base station is assigned a plurality of channels over which it can communicate with mobile units. A mobile unit within range of the base station communicates with the external world through the base station using these channels. Typically, the channels used by a base station are selected so that signals on any channel do not interfere with signals on another channel used by that base station.
In order to allow mobile units to transmit and receive telephone communications as the units travel over a wide geographic area, each cell is normally physically positioned so that its area of coverage is adjacent to and overlaps the areas of coverage of a number of other cells. When a mobile unit moves from an area covered by one base station to an area covered by another base station, communication with the mobile unit is transferred (handed off from one base station to another base station in an area where the coverage from different cells overlaps.
It is axiomatic that cellular telephone transmissions between the mobile units and the cells should be as free from interference as possible. The manner in which this is accomplished differs depending on the characteristics of the particular cellular system.
In the most prevalent American Mobile Phone System (AMPS) system, channels are defined by frequency. A frequency band providing approximately four hundred different adjoining FM frequency channels is allotted to each cellular system operator. In a typical AMPS system, each channel uses a fixed FM frequency band for downlink transmission from a base station to a mobile unit and another fixed FM frequency band for uplink transmission from a mobile unit to a cell. Typically, the frequencies assigned to the downlink transmissions for an entire AMPS cellular system immediately adjoin one another and are widely separated from the frequencies assigned to the uplink transmissions which also immediately adjoin one another.
Since channels are defined by frequency in an AMPS system, interference with any particular transmission is essentially due to transmissions on the same or immediately adjacent channels. To reduce this interference, an operator assigns channels to any single base station which are separated from one another in frequency sufficiently to eliminate interference between those channels. For example, an operator may allot to a base station a set of channels with frequencies which are each separated from the next by some large number (e.g., twenty-one) channels carrying intermediate frequencies.
Moreover, since a mobile unit in an AMPS system moving from an area covered by one base station to that covered by another base station must be transferred from one base station to the other in an area in which cell coverage overlaps, interference with base stations having overlapping cell coverage must also be eliminated. To do this, the channels allotted to the adjoining cells are carefully selected to eliminate the same frequencies. This is sometimes accomplished by assigning channels to a central cell which are widely separated in frequency in the manner described above, and then assigning channels to the cells surrounding that central cell using a pattern which increases each channel assignment by some number for each sequential cell surrounding the central cell. This produces what may be visualized as a honeycomb pattern of cells having a central cell surrounded by a number of overlapping cells transmitting on different frequencies. The same honeycomb pattern extends outward throughout the system with each cell surrounding the central cell functioning as a central cell surrounded by its own overlapping cells producing what is referred to as a reuse pattern. In such a pattern, interference on the same channel usually comes from cells at some distance from the cell carrying the useful information.
In most cellular systems, especially those with cells in urban areas carrying heavy traffic, a position at which a cell is situated includes two or three individual transceiving stations (referred to as xe2x80x9csectorsxe2x80x9d) each of which may include channels having the above-described frequency allotment of channels. The antennas of each sector are typically arranged to provide 180 or 120 degree coverage. The terms cells, sectors, and base stations are normally used interchangeably in this specification unless the context indicates otherwise. If an AMPS system includes significant numbers of sectored cells, six cells arranged in a honeycomb pattern surrounding a central cell may all be assigned different and theoretically non-interfering channels. However, outside the initial central cell and its immediately surrounding cells, the frequency reuse pattern requires that channels be replicated at much closer ranges than in a non-sectored system.
In another common type of mobile system called Time Division Multiple Access (TDMA), frequencies are assigned to the entire system in groups much like they are assigned in an AMPS system. However, within any frequency, each base station sends and receives in bursts during some number of different intervals or time slots. These time intervals within frequency bands then effectively constitute the individual channels. By using these intervals and assuring that the group of frequencies assigned to any individual base station differ from one another and from the frequencies assigned to base stations surrounding each individual base station, a channel reuse pattern is established which allows substantially greater use of the frequency spectrum because of the time division process.
A newer type of mobile system called Code Division Multiple Access (CDMA) uses encoded digital signals to transmit data. All of the base stations and mobile units of a CDMA system presently use the same xe2x80x9cspread spectrumxe2x80x9d frequency band of 1.25 megacycles to transmit the encoded digital signals although other band widths are presently proposed. The information bits of each transmission are expanded using coding information called a pseudo noise (PN) code. Each sector throughout a system uses the same PN code to encode the information transferred. Then each sector identifies itself by using a time offset (generally referred to as a pseudo noise (PN) offset) from some repeating initial time in the expanded transmission. Thus, one sector may begin an encoded transmission at the initial time, a second sector at an offset of one unit from the initial time, a third at an offset of two units, and so on up to a total of 512 offset units. Each transmission with a sector is placed on what is effectively a separate channel by further encoding the expanded transmission with one of a plurality of Walsh codes. A Walsh code is a mask used to encode and decode transmissions which eliminates transmissions sent using other Walsh codes. A transmission on a particular channel is decoded by applying a mask including the Walsh and PN codes to the received pattern of information bits commencing at the PN offset designated for the particular channel.
The CDMA system of transmission offers a number of advantages. One of these advantages is that a mobile unit may be receiving the same information from a number of different cells or sectors at the same instant. Since all transmissions take place on the same frequency band, a mobile unit actually receives all of the information which is available within its range. However, it only decodes information on channels which are directed to it. A CDMA mobile unit uses a receiver which is able to apply a number of decoding masks at the same instant to the entire spectrum of information which it receives. By knowing the Walsh codes and PN offsets defining channels which it desires to receive, a mobile unit may decode information from a single message sent to it by a number of different base stations simultaneously and combine that information to produce a single output message. Thus, while a signal from one sector may be fading, the same message may be received with adequate strength from another sector. This allows CDMA to offer the possibility of significantly better transmission.
In both AMPS and TDMA system, it is possible to reduce interference between channels by effecting frequency reuse plans in the manner described above. In theory, these forms of cell arrangement and channel assignments allows channel reuse patterns to be repeated at distances separated sufficiently to negate interference between mobile units on the same and adjacent channels.
Unfortunately, for a number reasons interference does occur in AMPS and TDMA systems even with well chosen frequency reuse plans. Antenna patterns, power levels, scattering, and wave diffraction differ from cell to cell. Buildings, various other structures, hills, mountains, foliage, and other physical objects cause signal strength to vary over the region covered by a cell. Consequently, the boundaries at which the signal strength of a channel falls below a level sufficient to support communications with a mobile unit vary widely within a cell and from cell to cell. For this reason, cells adjacent one another do not, in fact, typically form the precise geometric boundaries suggested above. Since cell boundaries must overlap to provide complete coverage of an area and allow handoff and because the boundaries of cells are imprecisely defined, signals will often interfere with one another even though they are generated by cells which are at distances theoretically sufficient to eliminate interference. This is especially true when a sectored cell pattern is used because the cells are much closer to one another than in a simple cell pattern.
In an AMPS system, a first signal on a channel from a remote cell interferes with a second (usually) stronger signal carrying a mobile transmission on the same channel within the coverage area of a cell when the drop in strength of the first signal from the second signal is less than some threshold level (typically 18 decibels). A signal from another cell on a channel at a frequency adjacent the frequency of a channel carrying a mobile transmission interferes when the drop in strength of the interfering signal from the serving signal is less than some second threshold level (typically 6 decibels).
Historically, in order to determine whether interference exists in an AMPS system, a mobile system operator relied on customer complaints. When customers register a sufficient number of complaints regarding communication at particular points in a system, an operator usually conducts a relatively expensive field test of the suspected portion of the system to measure signal strengths received from different cells. During the test, the portion of the system in which the tests are conducted is essentially disabled. Because of the expense and inconvenience, the tests are typically limited only to the suspected area. Because such tests are limited to determining the interference at those points at which a system operator expects to find interference, the efficacy of these tests is very suspect. A major problem with the process is that it does not provide a complete understanding of interference which actually exists in a system since typically only those positions at which extensive interference has been reported are tested. The process does not take into consideration all of the possible signals which might be propagating into the affected area to interfere with the carrier nor does it take into consideration the effects which a change in channel assignments may have in other areas of the system. Often (and possibly usually) this method of curing interference merely exports the interference to another portion of the system where it is only discovered when a sufficient number of complaints arise to warrant a field test of the newly isolated area of interference. Moreover, this method of eliminating interference is quite slow and labor intensive. Testing a medium sized system to eliminate interference may require as long as 400 man hours. The process greatly increases the costs without guaranteeing that interference will be eliminated. Because of the emerging nature of the market for cellular telephones, system changes which cause interference such as traffic growth are taking place constantly and at an accelerating rate.
Recently, a process has been devised by which the quality of service provided by an AMPS or TDMA system (and portions thereof) may be determined in terms of fixed verifiable quantities so that changes may be made to enhance the quality of service with an expectation that the changes will have the desired result in actually improving the quality of service provided by the system. The process utilizes data gathered during a drive of a service area during which transmitted signal strength and received signal strength at each location throughout the service area are obtained. These values provide actual data from which all locations at which interference may occur may be determined. Knowing the locations at which interference may occur allows values to be assigned to a particular service area by which an operator may quantify the quality of service and decide whether changes in the system are necessary. This process is described in U.S. patent application Ser. No. 08/887,101, entitled xe2x80x9cMethod of Improving the Operation of a Cellular Telephone Systemxe2x80x9d, E. Jensen et al, filed Jul. 2, 1997, and assigned to the assignee of the present invention.
Theoretically, in contrast to other types of systems, a CDMA transmission should be interference free throughout the system since data is decoded from digital information using masks which are supposed to eliminate interfering signals. However, in a CDMA system all transmissions are carried by bits transmitted on the same frequency spectrum. Because of this, information received by a mobile unit or a cell is effectively interference if the information is not directed to that particular receiver. That is, since a receiver receives all of the transmissions generated by any transmitter within range, the untranslated transmissions constitute interference in a CDMA system. Typically, before decoding, the desired transmission should have a strength not less than minus 14 dB when compared to the total strength of all transmissions being received. When the strength of the desired transmission falls below this point, the digital details of the message cannot be retrieved from the spectrum.
Encoding the signals provides a significant encoding gain because each bit of information is expanded by the pluralities of bits in each of the levels of coding. A decoded transmission of approximately 7 dB greater than interference present after decoding is just sufficient to provide signals of sufficient quality.
Because of the difference of the meaning of interference in the different types of cellular systems, the method of the above-mentioned patent application for quantifying the quality of service in AMPS or TDMA systems is not as useful when applied to CDMA systems. Consequently, interference in CDMA systems is typically eliminated at present by increasing the number of sectors when the transmissions with a sector increase beyond to a particular maximum number. However, it has been determined that such a criteria has very little to do with whether any particular sector is capable of handling additional transmissions or not. Adding sectors to a system is an expensive way of handling interference.
Consequently, it is desirable to provide a new process by which the quality of a CDMA cellular system may be quantified so that steps may be taken to improve the system.
The present invention is realized by a computer implemented process which determines all locations in a service area which are subject to interference-causing limitations, assigns an average service level to each such location, sums the service levels at all such locations, and divides the sum of the service levels at all such locations by the total service level for the service area to produce an interference value.
These and other features of the invention will be better understood by reference to the detailed description which follows taken together with the drawings in which like elements are referred to by like designations throughout the several views.